[Lowfer] Phasing exciters

[Ed Phillips]

Johan Bodin wrote:
> 
> Stewart,
> 
> > Generating a 90 degree phase shift (mathematically) is easy.
> > Google for "Hilbert transformers" and you'll see lots of
> 
> Another alternative is a complex filter. It takes care of the 90 degree shift
> and does bandpass filtering at the same time. My favorite.
> 
> Example:
> 
> 1.
> Design a simple FIR 1350 Hz lowpass filter.
> 
> 2.
> Make two sets of FIR coefficients by multiplying the original 1350 Hz LPF
> coeff's by "sin 1650 Hz" and "cos 1650 Hz" respectively (taking Fs into
> account). To preserve the phase linearity of the FIR, place the 1650Hz
> phase so that the coeff' sets become symmetrical or "antisymmetrical"
> with respect to the center of the filter (peak or zero crossing at the middle
> coefficient - or around the two middle coeff's if the FIRs have an even number
> of taps).
> 
> 3.
> Put the "sin FIR" in the "I" line and the "cos FIR" in the "Q" line and
> you have a single sided 300-3000 Hz response. Just swap I and Q
> to change sideband.
> 
> 73
> Johan SM6LKM

	Understand FIR filters, but excuse my ignorance, but how are you going
to implement it?  In software, I assume; if so, what does it run on?  If
so, where do you get the digital data?  From the sound card?  Way beyond
me I'm afraid.

	Everything is a lot simpler for someone who understands it than it is
for someone who doesn't.

Ed